This invention relates to compensating for image distortion on a pixel by pixel basis for the purpose of improving the quality of images captured by CCD camera systems.
Modern mail-handling procedures are becoming more sophisticated, and include such improvements as automated optical character (address) reading, and automated sorting and routing based on the results of such reading. A necessary aspect of such systems is the camera subsystem by which the address portion of each piece of mail is imaged for further processing. It has been found that line-array charge-coupled device (CCD) imagers, coupled with devices for scanning each mail piece before (in front of) the CCD array, are more effective than area-scanned imaging arrays. Such line scan arrays when combined with a scanning arrangement are known as scanners.
Objects or scenes scanned using digital CCD cameras exhibit some level of distortion attributable to irregularities in lighting and in the line array itself. This distortion is manifested as a distorted background over which the object information is superposed. The object information includes details which may be important to subsequent processing. The imperfections of the background of the image of the object may adversely affect that subsequent processing of the image. The imperfections of such cameras are important in the field of mail handling, and more generally in the fields of image information processing and optical character recognition.
Improved digital camera systems are desired.
An imaging system or camera according to an aspect of the invention includes a line discrete-pixel array, including a array of light-sensitive pixel sensors, for absorbing light during an interval and producing analog electrical signals in response to the absorbed light, and for transferring the signals to an output port. The resulting signals are representative of an image. The imaging system also includes an arrangement for illuminating an object to be imaged, and for causing illumination of the object to excite the array. This arrangement may include a light source directed toward the object, and an optical system associated with the imager for casting an image of a portion of the object onto the CCD line array. The image resulting from the image-representative signals may be subject to distortion arising from at least one of (a) pixel-to-pixel variations in sensitivity of the array, (b) image illumination variations, (c) channel errors in multichannel arrays, and (d) other sources, which result in distortion of an image generated from the signals. An arrangement is provided for scanning the object relative to the array, so as to produce a sequence of uncorrected signals associated with each pixel of the array across the scanned object. The scanning provides relative motion, so either the line array, the object, or both may be moved. A color-scale correction arrangement is coupled to the array, for receiving the uncorrected signals, and for correcting the signal representing each uncorrected pixel by a correction factor. In the case of a monochrome or black-and-white line array, the color-scale correction is a grey-scale correction. The correction factor is established or determined by a procedure including the steps of (a) with a particular color object (a white object in the case of a monochrome line array) before the array, averaging the values of the uncorrected signal for each of the pixels of the array over at least a portion of the scanned object, to thereby produce averaged uncorrected signals for each pixel of the line array, (b) for each of the pixels of the line array, determining an ideal averaged uncorrected signal value produced by an ideal pixel of an ideal imager. The correction factor which is applied to the signal from each pixel of the line array during normal operation is the factor required to bring the averaged uncorrected signal value to the ideal signal value. In a first type of correction, the correction factor is multiplied by the uncorrected signal from the pixel. In a second type of correction, the correction factor is added to the uncorrected signal from the pixel.
In a particularly advantageous embodiment of the invention, the imaging system further includes a threshold arrangement coupled to the color-scale correction arrangement or grey-scale correction arrangement, for enabling the color-scale correction arrangement when the uncorrected signal value for a particular pixel represents a color scale value or grey-scale value having a value lying on a first side of a threshold value, where the threshold value lies between black level and the ideal averaged uncorrected signal value for the particular color. Thus, in one embodiment, the correction of the signal from a pixel is not performed for that pixel if the signal level at the moment in question represents a value darker than the threshold value, whereas the correction is performed (by multiplication by, or addition of, the correction factor) if the signal value represents a grey or color value on the light or white side of the threshold.
A method according to another aspect of the invention, for correcting the signals produced by each pixel of a line-scan imager array, includes the set-up steps and normal-operation steps. The set-up steps include the step of scanning a white object or calibration target before (in front of) a line-scan array, to thereby produce, for each pixel of the array, a sequence of raw signals representing a portion of the object. The set-up further includes the step of averaging the values of the sequence of raw signals of each of the pixels across at least a portion of the object, to thereby produce an average value for each of the pixels, followed by determining, for each the pixel of the line array, one of (a) a multiplicative and (b) an additive correction factor which, when applied to the average value of the signal, results in a predetermined ideal value of the signal. Following the set-up steps, the method includes the normal-operation steps of, scanning an information-carrying object before the line-scan array, to thereby produce raw signals from each pixel of the line-scan array, and correcting the raw signals by one of (a) multiplying the raw signal from each pixel by the multiplicative correction factor and (b) adding to the raw signal from each pixel the additive correction factor, to thereby produce corrected signals for each of the pixel of the line-scan array. Following correction of the normal-operation signals, the corrected signals are processed to extract information therefrom, as by optical character recognition processes.
In an alternative mode of the method, an object of a uniform color is substituted for the white object.